Nanowire arrays are seeing increasing use in a variety of applications. See, e.g., U.S. Patent Application No. 20090256134. An exemplary silicon nanowire array might consist of a collection of silicon nanowires, on the order of 100 nm in diameter, on the order of several micrometers in height, and of approximately cylindrical or frustoconical shape. The axes of the nanowires run approximately parallel to each other. Each is attached at an end to a silicon substrate and is very roughly perpendicular to that substrate.
A silicon nanowire array on top of a silicon substrate, can alter the opto-electrical properties of the bulk silicon substrate. For example, a silicon nanowire array reduces the reflection of the silicon substrate, reduces the reflection at off-angles of incidence, and increases the absorption of the silicon in ways similar to traditional pyramids or light trapping mechanisms used in solar cells.
Some of the altered optical-electrical properties of silicon nanowires compared to bulk silicon are beneficial for solar cells. However, in order to form a solar cell, the two sides of a p-n junction need to be connected to the outside world. Unfortunately, contacting nanowires is not always easy.
One device design for nanowire solar cells places vertically aligned nanowires on top of a bulk (non-nanostructured) substrate. In this design, the back contact can easily be made from the backside of the substrate. The front contact, however, is more difficult to make.
The contact resistance increases the smaller the contact area. If contacts are made on top of the nanowire array, only the tips of the wires are in contact with the metal, and hence the contact resistance may be undesirably high. Too high contact resistance adversely impacts device efficiency.
For the types of solar cells currently manufactured, not using nanowire arrays, it is common to make contacts by screen printing. Screen printing is robust, has a high throughput, and is low-cost. The front and back contacts of a solar cell are typically formed in separate steps. For typical cell designs, silver is applied to the front, and aluminum to the back. For the front, paste is squeezed through a stainless steel or polyester fine metal mesh screen with an adjustable and finely controlled force delivered through metal or polymer squeegee. The screen defines a comb-like (finger line array and crossed bus bars) pattern designed to provide sufficient conductivity while minimizing optical shading from the metal lines. The paste is then dried at temperatures of 100-200 C to drive off organic solvents and fired at around 800 C to diffuse in the metal to establish a low contact resistance junction. For the back, an aluminum based paste is screen printed on the rear surface, establishing electrical contact and functioning as a back surface field. The aluminum is applied as a paste squeezed through a fine mesh screen, then fired at high temperatures to drive off organic solvents and diffuse in the aluminum to establish a low contact resistance junction. Although a continuous contact will result in lower resistance, commercial wafers utilize a back contact with an embedded mesh structure to reduce paste usage and minimize wafer warping during the subsequent high temperature processing steps. The pattern is defined in the screen by photolithography, although laser cut metal stencils may be utilized for smaller linewidths. Automatic screen printers are available that are capable of in-line, continuous operation with high throughput. These machines accept wafers from packs, cassettes or a belt line, place them with sufficient accuracy under the screen and deliver the printed wafers to the belt line. Detailed methods for screen printing are described in reference (1).
Electroplating processes have been developed as an alternative to screen printing for solar cell metallization. The process typically proceeds in two steps. In the first, a narrow groove is machined into a heavily doped region of silicon through any number of methods, including laser machining such as described by U.S. Pat. No. 4,726,850 or by other mechanical means. A metal seed layer such as nickel or copper is deposited, which immediately contacts the silicon and is chosen to have good mechanical and electrical contact to the silicon surface. In the subsequent step, the line is thickened by electroplating to increase the line conductivity. The second layer may be comprised of a distinct metal such as silver and the process parameters of each step may be chosen to optimize overall performance of the device. Areas in which there are no metal contacts may be covered with a lowly doped and passivated emitter. Linewidths for laser defined grooves for the metal seed layers may be 25-50 micrometers, reducing optical shading losses relative to screen printed methods. Alternatively, the silicon nitride layer is masked with photolithography, and etched through. The photo resist is removed and the SiN acts as a mask for electrodeposition onto the underlying silicon. Metal is then electrodeposited wherever the silicon nitride was etched. Unfortunately both these methods of electroplating require photolithography or expensive patterning techniques.
There is a need for improved techniques for making electrical contacts to nanostructured portions of a surface.